In the art of desk top publishing it is known to produce charts, graphics and illustrations by the selection and manipulation of graphics and text objects using a color monitor as a visulation tool and reference. Manipulation of objects includes the selection and/or adjustment of color. In this circumstance the visual appearance of the screen becomes the reference or original for reproduction. In other words, the colors that you see on the monitor are the colors that you want to see in the hard copy reproduction that is made by the system.
In the graphic arts it is known to reproduce a color image by scanning an original to produce a digital color image signal, processing the digital color image signal in a digital computer to produce a processed digital color image signal, and reproducing the color image from the processed digital image signal. This type of reproduction attempts to match the colors to the colors of the original color image. Although the present system can process original images by scanning negatives and other forms of hard prints it is directed more towards those applications where the image is first brought into existence by the operator interfacing with the computer and causing the image to be created by known color digital image processing techniques while using the color monitor to view the creation.
Attempts have been made to base the color digital image processing on a three color vision model of the human eye. In this so called tristimulus method, three color signals are determined from three linearly independent color mixture functions that are theoretically some linear function of the three color sensitivities of the human eye. In such systems, colors with the same tristimulus values, under the same viewing conditions, are perceived by persons with normal color vision as being identical. The goal has been to achieve a so called colorimetric or tristimulus match between an original and a reproduction, where the original is an original scene, a color CRT image, or an original photograph, and the reproduction is a CRT image or hard copy image produced by a color printer or color film scanner.
In one approach, as described in U.S. Pat. No. 4,409,614; issued Oct. 11, 1983 to Eichler et al., signals (E.sub.R E.sub.G E.sub.B) from a scanner, such as a color film scanner, are converted into three visual sensitivity based primary color signals (X,Y,Z). These signals may be subjected to a white balance for a D.sub.65 illuminant, and are then converted into coloring signals adapted to the basic coloring materials of the reproduction system. In one embodiment, the primary color signals are stored on a video tape recorder. Means are provided for modifying the adapted signals. The primary color signals from the recorder are subjected to the adaptation for the reproduction system and displayed on a color TV to show the effects of modifying the signals. When a desired modification has been achieved, the modified, adapted signals are supplied to a printer.
In another approach, as described in U.S. Pat. No. 4,500,919 issued Feb. 19, 1985 to Schreiber, tristimulus signals produced by a film scanner are displayed on a CRT without regard to any adaptation to any output medium. The tristimulus signals are subjected to aesthetic corrections by an operator, and the result viewed on the CRT. When the desired result is achieved on the CRT, the aesthetically corrected tristimulus signals are employed to produce corresponding reproduction signals to produce a colorimetric match in an output medium.
In both of the prior art approaches described above, there is an implicit assumption that the viewing conditions of the original and the reproduction will be identical. This is important since only then will a "colorimetric" match be equivalent to a visual appearance match. This result is due to the phenomenon known as "adaptation" whereby a human observer adapts to the color temperature of the ambient lighting conditions such that the visual appearance of different colors remains constant, even though the measured colorimetric values of the colors under the different illuminants change markedly. If a "colorimetric" match is made between an original image and its reproduction under different lighting conditions, the colors in the images will not appear the same to a viewer that is adapted to the different lighting conditions. Because of this, a considerable effort has been directed in the prior art to providing identical viewing conditions for an original and its reproduction. See for example U.S. Pat. No. 4,500,919 cited above.
The provision of identical viewing conditions however requires expensive hardware comprising light boxes and viewing booths and is not considered to be practical for the desk top publishing market. Furthermore, the prior art systems would at best. optimize the reproduced image for one set of viewing conditions but not for others.